1. Field of the Invention
This invention relates to conductive polymer compositions, methods of making such compositions, and electrical devices comprising such compositions.
2. Introduction to the Invention
Conductive polymer compositions and electrical devices comprising them are well-known. Such compositions comprise a polymer, and dispersed in the polymer, a conductive particulate filler. The type and quantity of the conductive particles, as well as the type of the polymer, influence the resistivity of the composition. Generally for compositions with resistivities greater than about 1 ohm-cm, carbon black is a preferred filler. For compositions with lower resistivities, metal particles are used. Compositions comprising carbon black are described in U.S. Pat. Nos. 4,237,441 (van Konynenburg et al), 4,388,607 (Toy et al), 4,534,889 (van Konynenburg et al), 4,560,498 (Horsma et al), 4,591,700 (Sopory), 4,724,417 (Au et al) , 4,774,024 (Deep et al), 4,935,156 (van Konynenburg et al), and 5,049,850 (Evans et al) Compositions comprising metal fillers are described in U.S. Pat. No. 4,545,926 (Fouts et al) and in U.S. application Ser. No. 07/788,655 (Baigrie et al), filed Nov. 6, 1991. The disclosure of each of these patents and pending applications is incorporated herein by reference.
In order to improve the electrical stability of conductive polymers it has been found that the addition of an inorganic filler such as alumina trihydrate is useful. Such compositions comprising carbon black, which are particularly useful for high voltage applications, i.e. exposure to voltages greater than about 100 volts, are described in U.S. Pat. Nos. 4,774,024 (Deep et al) and 5,049,850 (Evans et al). In addition, metal-filled compositions have been found to be more stable when a second filler, either another conductive filler such as a metal or carbon black or a nonconductive filler such as alumina trihydrate, is present. Such compositions are described in U.S. Pat. No. 4,545,926 (Fouts et al). These metal-filled compositions are designed to minimize resistance increase after exposure to high temperature conditions. High temperature conditions occur either as a result of cycling between an electrically powered and an unpowered state, or as a result of passive thermal treatment. Despite the objective of maintaining a relatively constant room temperature resistance after such exposure, these metal-filled compositions often do increase in resistance on cycling. Furthermore, it is difficult to make them reproducibly at a given low resistivity value.